The role of fission in the The role of fission in the r-process r-process

nucleosynthesis nucleosynthesis Aleksandra Kelić

GSI – Darmstadt

Together with: Karl-Heinz Schmidt, Karlheinz Langanke

GSI – Darmstadt

Nikolaj ZinnerUniversity of Århus - Århus

MotivationMotivation

S. Wanajo et al., NPA in print

- Fission could have influence on the Amax of the r-process as well as on the yields of transuranium elements and, consequently, on the determination of the age of the Galaxy and the Universe [1]. In cases where high neutron densities exist over long periods, fission could also influence the abundances of nuclei in the region A ~ 90 and 130 due to the fission cycling [2,3].

[1] J.J. Cowan et al, Phys. Rep. 208 (1991) 267 [2] P.A. Seeger et al, APJ. 11 Suppl. (1965) S121 [3] T. Rauscher et al, APJ. 429 (1994) 49

What do we need?What do we need?

• What are the needed ingredients?– Fission barriers– Mass and charge division in fission- Fission dynamics

• Importance of nuclear structure at large deformations

• And all this for heavy, very neutron-rich nuclei

Covered in this talk

How well can we describe fission?How well can we describe fission?

Empirical systematics - Problem is often too complex

Theoretical models - Way to go! - But, not always precise enough and still very time consuming

- Encouraging progress in a full microscopic description of fission: H. Goutte et al., PRC 71 (2005) 024316 J.M. Pearson and S. Goriely, NPA in print

Semi-empirical models - Theory-guided systematics

Saddle-point massesSaddle-point masses

Open problemOpen problem Limited experimental information on the height of the fission barrier

Neutron-induced fission rates for U isotopes

I. Panov et al., NPA 747 (2005) 633

IdeaIdeaPredictions of theoretical models are examined by means of a detailed analysis of the isotopic trends of ground-state masses and fission barriers.

Usad Empirical saddle-point shell-correction energy

)(expexp macrof

macrofsad EMMEU

Macroscopic saddle-point

mass

Experimental saddle-point

mass

M. Dahlinger et al., NPA376 (1982) 94

Studied modelsStudied models

1.) Droplet model (DM) [Myers 1977], which is a basis of often used results of the Howard-Möller fission-barrier calculations [Howard&Möller 1980]

2.) Finite-range liquid drop model (FRLDM) [Sierk 1986, Möller et al

1995]

3.) Thomas-Fermi model (TF) [Myers&Swiatecki 1996, 1999]

4.) Extended Thomas-Fermi model (ETF) [Mamdouh et al. 2001]

W.D. Myers, „Droplet Model of Atomic Nuclei“, 1977 IFI/PlenumW.M. Howard and P. Möller, ADNDT 25 (1980) 219.A. Sierk, PRC33 (1986) 2039.P. Möller et al, ADNDT 59 (1995) 185. W.D. Myers and W.J. Swiatecki, NPA 601( 1996) 141 W.D. Myers and W.J. Swiatecki, PRC 60 (1999) 0 14606-1A. Mamdouh et al, NPA 679 (2001) 337

ResultsResultsSlopes of δUsad as a function of the neutron excess

The most realistic predictions are expected from the TF model and the FRLD model. Inconsistencies in the saddle-point mass predictions of the droplet model and the extended Thomas-Fermi model.

Mass and charge division in fissionMass and charge division in fission

Measured fission-fragment Z distributionsMeasured fission-fragment Z distributions

K.-H. Schmidt et al., NPA 665 (2000) 221

Experimental survey by use of secondary beams of radioactive isotopes

Macroscopic-microscopic approachMacroscopic-microscopic approach

238U 208Pb

V V- For each fission fragment we get:

- Mass- Charge- Velocity- Excitation energy

- Transition from single-humped to double-humped explained bymacroscopic (fissionning nucleus) and microscopic (nascent fragments) properties of the potential-energy landscape near the saddle point.

82 N=82 N~9090

Comparison with existing dataComparison with existing data

89Ac

90Th

91Pa

92U

131

135

134

133

132

136

137

138

139

140

141

142

Fission of secondary beams after the EM excitation:black - experimentred - calculations

Application in astrophysics - first stepApplication in astrophysics - first step A and Z distributions in neutrino-induced fission of r-process nuclei 1. -nucleus interaction RPA2. Deexcitation GSI code ABLA

Phys. Lett. B616 (2005) 48

N. Zinner

How to continueHow to continue

Detailed r-process network calculations (N. Zinner and D. Mocelj)

New experimental data- Mass AND charge distributions of both fission fragments at different, well defined excitation energies- Light particles and gammas emitted in coincidence

FAIR facility at GSI

Close collaboration between experiment and theory

Microscopic approach to fissionMicroscopic approach to fission H. Goutte et al., PRC 71 (2005) 024316HFB + time-dependent generator coordinate method:

How well do we understand fission?How well do we understand fission?

Influence of nuclear structure (shell corrections, pairing, ...)

M.G. Itkis et al., Proc. Large-scale collective motion of atomic nuclei, Brolo, 1996

K.-H. Schmidt et al., NPA 665 (2000) 221

Comparison with dataComparison with dataHow does the model work in more complex scenario?

238U+p at 1 A GeV

Example for uraniumExample for uranium

Usad as a function of a neutron number

A realistic macroscopic model should give almost a zero slope!